—
416
17.
SMALL CURRENT TECHNIQUE
be no deection when the needle voltage is applied, particularly
if the needle is tilted or bent out of shape.
In practice, the
deections are reduced to as small a value as possible, say a fraction of one millimeter on a scale one meter away.
—
If the zero position keeps changing (drifts), tighten the suspem
‘
sion screws, resolder the suspension joints or put in a new one.
4. Apply a small voltage to the quadrants (v = 1 volt). If
the deection creeps
back to zero, clean the insulators with
Voltage Calibration—1. A convenient arrangement for applying various potentials _is indicated in gure 17—29. Four or ve
dry cells are connected across a high resistance box (10,000 ohms
or more).
The total voltage E is read on a voltmeter. A plug
is used to tap off a known voltage, @ : rE/R.
2. Observe the deection obtained
for various potentials
applied to one quadrant, With a xed needle potential (say 15
volts).
Deections up
to 20 or 25 centimeters on each side of the
zero pdsition should be used.
3. Repeat (2) using, in each case, needle potentials of 30, 60,
120 volts.
4. Plot the various values of quadrant potentials, positive and
negative, as abscissae and the deections from zero as ordinates
for
needle potentials. Draw all curves on the same
chart With the zero point in the center of the page.
Note the
range over Which the deections are directly proportional to the
quadraht potentials.
5. Plot deeçtions as the ordinates and needle potentials as
abscissae, maintaining a denite potential on the quadrants
_
(say 2 volts). Note the deviation from a straight line and state
on the curve whether the distortion couple is aiding or opposing
th€mechanical couple.
.
6. Compute the voltage sensitiveness of the instrument under
the most favorable condition, recording its value on the chart of
-
"part 5.
-
_
_
Pr‘ecautiom.—See that the various parts ofïthe electrometer
connected so as to have denite relative potentials.
.
not fail to include the protective resistance P in all of the
Connections as indicated.
'
_
416
17.
SMALL CURRENT TECHNIQUE
be no deection when the needle voltage is applied, particularly
if the needle is tilted or bent out of shape.
In practice, the
deections are reduced to as small a value as possible, say a fraction of one millimeter on a scale one meter away.
—
If the zero position keeps changing (drifts), tighten the suspem
‘
sion screws, resolder the suspension joints or put in a new one.
4. Apply a small voltage to the quadrants (v = 1 volt). If
the deection creeps
back to zero, clean the insulators with
Voltage Calibration—1. A convenient arrangement for applying various potentials _is indicated in gure 17—29. Four or ve
dry cells are connected across a high resistance box (10,000 ohms
or more).
The total voltage E is read on a voltmeter. A plug
is used to tap off a known voltage, @ : rE/R.
2. Observe the deection obtained
for various potentials
applied to one quadrant, With a xed needle potential (say 15
volts).
Deections up
to 20 or 25 centimeters on each side of the
zero pdsition should be used.
3. Repeat (2) using, in each case, needle potentials of 30, 60,
120 volts.
4. Plot the various values of quadrant potentials, positive and
negative, as abscissae and the deections from zero as ordinates
for
needle potentials. Draw all curves on the same
chart With the zero point in the center of the page.
Note the
range over Which the deections are directly proportional to the
quadraht potentials.
5. Plot deeçtions as the ordinates and needle potentials as
abscissae, maintaining a denite potential on the quadrants
_
(say 2 volts). Note the deviation from a straight line and state
on the curve whether the distortion couple is aiding or opposing
th€mechanical couple.
.
6. Compute the voltage sensitiveness of the instrument under
the most favorable condition, recording its value on the chart of
-
"part 5.
-
_
_
Pr‘ecautiom.—See that the various parts ofïthe electrometer
connected so as to have denite relative potentials.
.
not fail to include the protective resistance P in all of the
Connections as indicated.
'
_